Fuel control for a regenerative gas turbine engine



Jan. 26, 1965 J. M. MALJANIAN ETAL 3,166,902

FUEL. CONTROL FOR A REGENERATIVE GAS TURBINE ENGINE Filed Nov. 15, 19626 Sheets-Sheet l FIGJ /3 INVENTORS JOHN M. MALJANIAN GENE. A.. MEYER BYATTORNEY Jan. 26, 1965 J. M. MALJANIAN ETAL 3,166,902

FUEL CONTROL FOR A REGENERATIVE GAS TURBINE ENGINE Filed Nov. 15, 1962 6Sheets-Sheet 2 ap/z/z/v IN ENTORS JOHN 1 M. MALJANIAN GENE. A- MATTORNEY Jan. 26, 1965 J. M. MALJANIAN ETAL 3,166,902

FUEL. CONTROL FOR A REGENERATIVE GAS TURBINE ENGINE Filed Nov. 15, 19626 Sheets-Sheet 5 FIC5 3 C F24" 7/4 74 MM 7 7f 7% m/a /a/ 4% i KNNZ)INVENTORS JOHN M- MALJANIAN GENE A- MEYER Jam 1965 J. M. MALJANIAN ETAL3,

FUEL. CONTROL FOR A REGENERATIVE GAS TURBINE ENGINE Filed Nov. 15. 19626 Sheets-Sheet 4 BYW ATTORN Y Jan. 26, 1965 J. M. MALJANIAN ETAL3,166,902

FUEL. CONTROL FOR A REGENERATIVE GAS TURBINE ENGINE Filed Nov. 15, 19626 Sheets-Sheet 5 INVENTORS JOHN M- MALJANIAN GENE A- MEYER ATTORNEY Jan.26, 1965 Filed Nov. 15. 1962 FIG--8 J. M. MALJANIAN ETAL 3, 66,902

FUEL. CONTROL FOR A REGENERATIVE GAS TURBINE ENGINE 6 Sheets-Sheet 6FIC3-9 woawsm/ ACZZZEPAI'E 567/50046' /&0Z awa $7275 BYW ATTORNEY UnitedStates Patent 3,166,902 FUEL CONTROL FOR A REGENERATIVE GAS TURBINEENGINE John M. Maljanian, 'Newington, and Gene A. Meyer, 2 Simsbury,Conn., assignors to Chandler Evans Corporation, Hartford, Conn., acorporation of Delaware Filed Nov. 15, 1962, Ser. No; 237,914 31 Claims.(Cl. 60-3916) This invention pertains to a fuel control system foroperation of complex, multiple cycle, twin-spool, gas turbine engineswhich produce low specific fuel consump tions of orders of magnitudethat are comparable to compression ignition type (diesel) engines; andmore particularly has reference to such engines in which there isincorporated a heat exchanger that may be of a recupera- .tive orregenerative type, for which corrections are made in the scheduled fuelflow to the engine, to compensate for the heat added to the fuel by theheat exchanger, in order to prevent engine surge and/ orovertemperatures.

As with all gas turbine engines, fuel is scheduled in such a manner asto prevent engine surge and/or overtemperature, which cause loss ofperformance and possible structural damage. Turbine overtemperaturecauses damage by exceeding the temperature capability of its materialswhich results in shorter engine life and possibly destruction of theengine during operation.

The required steady state operating fuel flow to the engine varies as afunction of engine inlet temperature, altitude, compressor pressureratio, engine speed, and engine inlet ram conditions; which entitiesconstitute the,

control parameters that determine the fuel flow to the engine.

Acceleration fuel flow is scheduled to provide fast accelerations fromone operating speed to another, as well as rapid decelerations where thepower demand is varied from one speed setting to another. .These changesin speed setting which produce a corresponding change in power outputmust be accomplished without causing engine surge, overtemperature, orany other conditions which, as a result of fuel flow variance fromacceptable limits, could cause damage to the engine.

The general problem associated with the control of gas turbine engineswith heat exchangers is the adjustment in fuel flow to the engine whichis required as a result of conditions brought about by the recuperator,since heat added from the recuperative or regenerative type heatexchanger (hereinafter referred to simply as the recuperator) must becompensated for by a corresponding correction in fuel flow.

- tanks) .or power stations (generator set, etc). ever, due to weightand size problems associated with Accordingly, one of the primaryobjects of our invention is to provide a fuel and speed control systemhaving means for making the necessary correction in fuel flow to theengine by sensing the temperature of the recuperator and correcting thefuel flow in the fuel control as a function of this temperature.

Due to the complex cycle of the engine being controlled, it is importantthat the ratio of selected corrected speeds of the two component spools(i.e. high and low pressure compressor rotors) be maintained at adefinite value for each particular selected position of the engine powercontrol lever. Since no mechanical drive is available as a measure oflow pressure rot-or speed, the low pressure compressor dischargepressure is used as a measure of this speed. High pressure speed ismechanically available and is used directly.

Accordingly, another primary object of our invention is to provide afuel and speed control system having means for maintaining the ratio ofselected corrected speeds of the high and low pressure compressor rotorsat a definite value, for each particular selected position of the engine3,166,902 Patented .lan. 26, 1965 power controllever. More specifically,for each power lever position, a given value of high pressure rotorspeed is selected, along with a corresponding steady state dischargepressure (P of the low pressure compressor. Automatic compressor inlettemperature (T correction adjusts the absolute values of these settingsfor the particular T condition, to provide the necessary steady statesettings which achieve optimum fuel consumption rates, at optimumconditions of the recuperator, without compressor surge and turbineovertemperature.

Still another object of our invention is to provide a fuel and speedcontrol in which the torque output of the free power turbine of theengine is controlled by controlling the corrected speed (N T of the highpressure compressor rotor, so that the torque exerted by the powerturbine is independent of the temperature (T of the air entering theengine, as shown later in this specification.

Other objects of our invention are to provide a fuel and speed controlsystem embodying the following novel features:

(1) A fuel control apparatus which is contained in a single, unitarycontrol package, with single lever for operation under all conditions ofenvironment, speed altitude, etc.

(2) Means to provide for automatic starting of the engine.

(3) Means to automatically provide for maximum acceleration of theengine from one power lever setting to another, without causing engine(compressor) surge and/ or engine overtemperature. 1

(4) Means to provide steady state operation of the (5) Means toautomatically provide for maximum deceleration of the engine fromanygiven power lever setting to any lower power leversetting, withoutcausing burner blowout.

(6) Means to provide automatic temperature compensation for highpressure rotor load torque, since provision can be made on the enginefor extraction of power from the high pressure rotor;

(7) A control system which is adaptable to gas turbines for eitherairborne or ground vehicles (trucks,

How-

the aircraft-engine, the engines equipped with recuperators (heatexchangers) would be more suitable to the ground. applicationsalthoughthe principles ofcontrol of our invention also apply to engines whichcould be used in aircraft.

Further objects of our invention are to provide an improved fuel andspeedcontrol apparatus for a twinspool engine, equipped with arecuperator, which embodies the following noval features:

(8) A control apparatus comprising, in a single selfoontained package, aprimary fuel supply and control system, and a secondary fuel supply andcontrol system which supplements the primary system; each systemcomprising a series of component coordinated hydraulic devices forregulating fuel delivery to'the engine; said devices being collectivelyresponsive to the control parameters specified hereinbelow, and subjectto a single manual control power lever.

(9) A control apparatus which comprises a series of devices that measureinlet air absolute temperature,'high pressure compressor dischargepressure, recuperator temperature, and engine speed (rpm), and positionsa pri mary fuel metering valve in accordance with a preselectedcomposite function of said temperature, said pressure and 3 speed; whilethe pressure differential (metering head) across said valve ismaintained at a constant selected value.

() A fully automatic hydraulic control apparatus in which the primaryfuel flow to the engine is compensated for variations in absolute inletair temperature, high pressure compressor discharge pressure,recuperator temperature, and engine speed; and said compensation beinginherent in the operation of the apparatus, so that additionalcorrection factors for these variables are not required,

, in order to compensate for variations'in operating conditions due tosaid variables. I

(11)' A fully automatic, hydraulic control apparatus 'which uses'ascontrol parametersf jfor limiting the maximum fuel fio w to the engine,the quantities corrected speed and corrected; fuel new," as definedhereinbelow.

(12) A control apparatus which'produces a substantially correctedconstant engine speed, (N /V11) correspondingto any selected position ofasingle manual control lever, under all engine operating conditions. I

(13) A control apparatus which functions so that the engine can beaccelerated at a maximum rate, corresponding to the temperatureof theair entering the engine (lowpressure compressor), without causingcompressor stall and decelerated at a maximum rate without causingburner .blowout. I

(1 4)'A control apparatus wherein the fuel flow regulating mechanismoperates 'in itsownfluid (whichmay'be either an oil oi' engine fuel),and acts directly on thefuel I supplied by a constant delivery pump andregulates its flow to the engine by means of a plurality of suitablecontrol valves. I II I I I I I (15) A control apparatus "havingoverridespeeds and l temperature control devices which prevent the"engine from operating at'excessive speeds and temperatures.

With these and other objects in'view of which may be incidentitoour-improvements, our'invention consists in the combination "andarrangement of elements hereinafter-described and illustrated intheaccompanying drawings, in

which FIG. 1' is a schematic diagram of our improved fuel supply andcontrol system as applied to a twin spool turbirie engine having anincorporated recuperator.

FIG. 2 is a diagrammatic section of the engine of FIG. 1, showing theconnections-betweedthe engine and other elements of the -,fuel supplyand control system of our invention. I

FIG. 3 is a schematic sectional-view of a fuel and speed "edntrolapparatus embodying the principles and novel features of our invention.

' FIG.-4-is a schematic diagram of a simplified version of ourinvention, as hereinafter more particularly described.

FIGS. 5-11 are diagrams of certain operating characteristics of theengine and control' shown in FIGS. 1 4.

A'sshown in FIG. 1, the engine, to which our improved .fuel supply andcontrol 'syste'm' isapplied; comprises a low-pressure air compressor Cdriven by a secondary gas turbine T arranged in air flow seriesrelation-to a highpressure compresSorC,- drivenby afprimary gas turbineT The rotor of the low-pressure compressor C directly connected toft-lie'rotor ofauxiliary turbine T and the rotor ofthehigh-pressurecompressor C directly from compressor Cwand is supplied-with fuel (oil),at r a flow rate W from the fuel control unit shown in FIGS. 2.and 3.Thegases generated by the combustion of fuel in chamber.B.flowthroughprimarylturbine T to a secondary combustion chamber B towhich additional fuel is supplied, at a rate W from the fuel controlunit shown in FIGS. 2 and 3. The gases, generated by the combustion offuel in chamber B augment the gases flowing into said chamber fromturbine T and both flow though the free power turbine PT and thesecondary turbine T to a recuperator RC, from which they are exhaustedinto the outside air. Compressed air flowing from compressor C toprimary combustion chamber B also passes through the recuperator RCwhere it receives heat from the exhaust gases flowing therethrough.

interposed in air flow series relation between low-pressure compressor Cand high-pressure compressor C is an intercooler 1C, which receivescooling fluid from a source (not shown), and cools the air flowing fromcompressor C to compressor C Air enters the engine from thecircumambient atmosphere (station Q) through air flow silencer S andflows successively through compressor C intercooler IC, compressor Crecuperator R C, to primary combustion chamber 13,; and the gasesgenerated, in chamber B flow through primary turbine T to secondarycombustion chamber, where they are augmented by the gases generated inchamber B and both then flow successively through power turbine PT,auxiliary turbine T and recuperator RC, from which they are exhaustedinto the circumambient atmosphere.

The underscored numerals Q to l indicate the successive stations in thefluid flow path from air inlet station 9 to exhaust gas outlet station las clearly indicated I in FIG. 1.

Broadly comprehended, the fuel supply and control system of ourinvention, as herein disclosed, comprises the concept of .ahydromechanical fuel metering system which schedules both primary andsecondary engine metered fuel fiow in accordance with requirements forsatisfactory engine operation.

The inputs (i.e. control parameters) of our control system are:

( l) High-pressure rotor speed, N

(2) Low-pressure compressor pressure, P

(3) High-pressure compressor pressure, P

(4) Compressor inlet temperature, T

(5) Cornbustor inlet (recuperator) temperature, T

(6) Power turbine speed, N

(7) Power lever angle, PLA.

(8) Primary fuel flow to the engine, W

(9) Secondary fuel How to the engine, W

The outputs (i.e. controlled entities) of our control system are: I

(1) Primary fuel flow W regulation in accordance with engine performancerequirements.

'(2) Secondary (or reheater fuel fiow), W regula tion in accordance withengine performance requirements.

(3) Secondary augmentation fuel flow, W L, regulation in accordance withengine performance requirements.

by controlling P pressure as a function of set power lever angle, PLA.As with speed, N set values of pressure, P are corrected or reset with Ttemperature, to main-. tain essentially corrected N speed settings(since P is a measure of N or low-pressure rotor speed).

The selected power lever angle, PLA, sets required cor-- respondingvalues of both speed N and pressure P simul- 'ating system is a part ofthe main control.

taneously, theT temperature correction being automati cally applied toboth settings by a T transducer. An adjustable hydraulic dashpot isprovided on the throttle shaft between the P and N setting to delayreturn of the settingrin the deceleration-direction, and tlfus preventlow-pressure compressor surge during decelerations. The dashpotis-ineifectiveduring increased settings of the power lever.

As is requi-red by desired engine performance, secondary fuel flow W isaugmented by an increment of fuel flow W which is proportional to theamount which the N governor has cut in, or a fixed value, whichever isthe least.

The power turbine (PT) governor. generates a pressure signal (OS) as afunction of power turbine speed, N At power turbine cut-in speed, thispressure acts to reduce both primary and secondary fuel flow as afunction of power turbine speed error to effectively serve the samefunction asretarding the throttle. The pressure gener- Overspeed signal(OS) is available in the form of the generated pressure. A pressureswitch (not shown), set for the pressure corresponding to the shutdownspeed, supplies an electrical signal for shutting down the engine ifrequired.

As shown in FIG. 3 the fuel and speed control package contains thefollowing main elements:

(i) Positive displacement fuel pump integral with the control package;

(ii) Primary fuel metering and speed control system for the highpressure rotor;

(iii) Secondary fuel metering system and compressor discharge pressurecontrol system for the low-pressure rotor;

(iv) Single power lever mechanism to set the prescribed combination ofhigh-pressure rotor speed and low-pressure rotor pressure; P

(v) Power turbine governor to reduce both primary and secondary fuelflow to the engine as a function of power turbine speed error.

(vi) Modification of fuel flow with variations of recuperator gastemperature, so that the primary acceleration fuel flow will beproportional to the product of a function of high-pressure compressordischarge pressure (P and a function of recuperator temperature (T (vii)Compressor inlet temperature biasing of the highpressure rotor speedselector, so that the selected speed is essentially a corrected speed (N/T (viii) Time delay on deceleration of the high pressure rotor.

(ix) Compressor inlettemperature biasing of the lowpressure rotorpressure selector.

The two-spoo engine with which our invention operates requires twoseparate but cooperating fuel control systems, one for each of the tworotors. Each system must provide independent means for transient andsteady state control of the rotor with which it is associated, togetherwith certain additional coordinating functions;

The, engine with which our invention operates is also distinguished fromconventional aircraft gas turbines by the presence of a recuperator,which introduces a special problem into the construction and functioningof the control; in that-the control must modify the maximum limiting, oracceleration, fuel schedule to compensate for the heatrecovered from therecuperator; and this is accomplished by regulating the primary fuelflow in accordance with the equation:

where A A and A are design constants.

The multiplication on the right-hand side ofth-is equation may becarried out by a combination of variable and fixed orifices in series incertain fuel flow conduits.

The recuperator gas temperatures. (T5) of the order of as high as 1400"F. must be sensed'for usein the control.

Liquid and gas filled bulbs are not suitable, since it is difiicult toprevent permeation of the bulb walls by the fluid at such a hightemperature, with a consequent error incalibration. We therefore willuse a differential metal expansion device to respond to the recuperatortemperature. The sensed expansion will be transduced to a hydraulicpressure for use in the control.

Metering of acceleration limit fuel, both primary and secondary, isaccomplished by applying a fixed metering head to avalve whose port areais varied. in proportion to the control parameters. Governing in steadystate, for both rotors, is done by a proportional closed loop errorsensing control, in which the desired operating variable (rotor N for.the high-pressure rotor, and compressor discharge pressure P2 for thelow-pressure rotor), are biased by inlet air temperature and areset by amanual power lever; and the governors act to vary fuel flow inproportion to the error between the actual and desired values of thevariable.

Means are provided, when the N governor cuts in, to augment the fuelflow to the low-pressure rotor by an amount equal to the reduction of Nrotor fuel. There is also a dashpot provision to delay deceleration ofthe N rotor after throttle chop.

A power turbine governor will proportionally reduce fuel flow to bothcombustors when a set speed is exceeded. A hydraulic pressure signalwill be available to actuate an engine shut-down device when a setabsolute maximum speed is reached.

Referring now to FIG. 2 of the drawings, there are shown, as principalelements of the engine mentioned hereinabove: a supporting body 1, anair inlet 2 leading to a low-pressure compressor G which is connected bya shaft 3 to a low-pressure (secondary) gas turbine T Compressed air isdischarged from compressor C through an intercooler 4, to ahigh-pressure compressor C where its temperature is increased from T toT and its pressure is increased from P to P Intercooler 4 is providedwith a fan 5, driven by countershafts 6--and 7 from shaft 3, to

circulate cooling fluid around theair passages through said intercooler,and lowers the temperature T of the air passing from compressorC to atemperature T of the air entering compressor C Compressor C is connectedby a shaft 8 to a high-pressure gas turbine T and dischargesits airthrough a recuperator RC, where the air isheated to a temperature T byexhaust gases from low-pressure turbine T flowing through a connectingconduit After passing through recuperator RC, these gases are dischargedthrough a passageway 10 into the outside air. A temperature sensor 11senses the temperature T in the recupenator RC, as more fully describedhereinbelow. Low-pressure compressor C turbine T and connecting shaft 3constitute the outerspool of the engine; while high-pressure compressorC turbine T and connecting shaft 8, constitute the inner spool of theengine.

The compressed air passing through recuperator RC flows intoprimarycombustion'chamber B which receivesfuel oil, through a conduit 12from a fuel control unit 13, and the resulting combustion of said fuelin chamber B generates gases that flow under increased temperature andpressure, through high-pressure turbine T into secondary combustionchamber B Additional fuel oil is fed into chamber B from control unit13, through a connecting conduit 14, and the additional cornbustionofthis fuel in chamber B increases the pressure and temperature of thegases therein. From chamber B the combustion gases flow successivelythrough a free power gas turbine PT, low-pressure turbine T andrecuperator RC. The free power turbine PT generates the useful power ofthe engine 1 which is transmittedby a shaft 15 to the driven load (e.g.wheels of a ground vehicle, or the propeller of a motor boat oraircraft).

The fuel control unit 13 is connected to a fuel supply tank 16, byasupply conduit 17, andan excess fuel return conduit 18. Unit 13 is alsoconnected to the engine by: a conduit 19, which communicates with thedischarge passage 20 of high-pressure compressor C by a conduit 21,which communicates with a temperature bulb 22 located in air inlet 2;and by a conduit 23 which communicates withthe discharge passage 24 oflow-pressure compressor C Shafts 25 and 26, connected to the shaft 8,drive a high-pressure rotor speed governor in control unit 13, andshafts 27, 28 and 29, connected to the power turbine shaft '15, drive apower turbine speed governor, also in control unit 13. Temperaturesensor 11 is connected to control unit 13,- by conduits 43 and 78, whichtransmit the control pressure (P,,) in unit 13 to said sensor.

SIMPLIFIED VERSION OF ENGINE In order to facilitate an understanding ofone of the principal problems solved by our invention, we have depictedin FIG. 4 a simplified version of a free power gas turbine engineequipped with a recuperator. From a comparison of FIGS. 1 and 4, it willbe noted that, in the simplified engine of FIG. 4, the low-pressurecompressor C and its driving turbine T as well as the secondarycombustion chamber B (of FIG. 1) have been eliminated, and the exhaustgases from the high-pressure turbine T pass directly to the free powerturbine PT and are then returned directly to the recuperator RC.

Accordingly, only the following stations in the fluid flow path areconsidered, in FIG. 4: Stations 1 and g at the entrance and exit of thehigh-pressure compressor C station at the junction of the recuperator RCand combustion chamber B station 2 at the junction of chamber B and thehigh-pressure turbine T station E between turbine T and the free powerturbine PT; station Q between turbine PT and reouperator RC; and station'i at the exit of the recuperator.

On the simplified basis of FIG. 4 just indicated, the following is ananalysis of the functions and operation of the elements involved.

Air is drawn into the compressor at station 1 at pressure P andtemperature T and leaves the compressor rotor at station 2 at a higherpressure and temperature P2 and T and the air from station 3 passesthrough the recuperator RC to station t. In the recupcrator RC, itreceives heat from counter-flowing hot gas from the power turbine PTexhaust.

At station :3 the air, which has now been heated to temperature T entersthe combustion chamber 3;, where additional heat is added by burningfuel, (W In the combustion chamber the air reaches a temperature T, atstation where it enters primary turbine T The heated air expands throughthis turbine, doing work thereon, and leaves it at station 5 with thepressure and temperature P and T The primary turbine T is coupled to thecompressor, and the combination of these two, with the recuperator RCand combustion chamber B is referred to as the gas generator portion ofthe engine.

The gas from the gas generator, at pressure P and temperature T enters asecond and independent power turbine PT at station Q and expands to anexhaust pressure and temperature P and T at station During expansionthrough the free turbine PT, usually referred to as a free powerturbine, useful work is done, and this work may be extracted through areduction gear coupled to the power output shaft.

The exhaust gas from station 6 is now passed through the recuperator RC,where it gives up heat to the counterflowing compressed air from thecompressor. The spent gas is finally exhausted from the engine atstation 2.

There are two particular problems that arise in the control of such arecuperativc gas turbine engine.

The first problem is this: the fuel flow to the combustion chamber mustbe modified according to the temperature of the air leaving therecuperator at station 3.

Structural and metallurgical limitations impose a top limit on thetemperature of the gas entering the turbine at station g, and thistemperature is attained in three steps, starting with the inlet air atstation 1, namely (1) The temperature rise (T -T due to compression ofthe air,

(2) The rise (1 3-1 due to heat absorbed from the recuperator, and

(3) The rise (T -T due to combustion of fuel.

So that the permissible top limit of the temperature T; shall not beexceeded it is therefore necessary that the control modify the fuel flowaccording to recuperator gas temperature T This could be achieved quitewell if the temperature T 4 could be measured directly, and the fuelflow W controlled so that the allowable maximum of T is never exceeded.This, however, is impractical, since for one reason, T, is of the orderof 1800" F. or more, and most measuring devices are not very reliable atsuch temperatures. A more important reason is that the characteristicsof the engine are such that almost instantaneous response of the fuelflow is needed when T, is used as a measured control parameter, and alltemperature measuring devices have time lags that are intolerably long.

However, we have discovered that the turbine T can be protected againstovertemperature without measuring T As disclosed hereinbelow, a methodis disclosed whereby a measurement of the lower temperature T can beused, along with other control parameters, to control the fuel fiow W insuch a way that the turbine gas temperature T 4 is limited to a maximumpermissible value. Our method also avoids the need for very rapidresponse in the temperature measuring device, since the recuperatoritself has a rather long time constant. Our temperature measuringdevice, as described hereinbelow, has an anticipating characteristic, sothat the effect of time lags is even further minimized.

The second problem referred to above has to do with the torque output ofthe free power turbine PT. As shown hereinafter, when the gas generatorportion of the engine is rotating at a speed N r.p.m., and thetemperature of the incoming air is T degrees Rankine, and if the freepower turbine is locked at standstill, the torque Q exerted by the freepower turbine is a function of the corrected speed (N /Vi).

As the temperature of the incoming air may vary from 65 degrees F. todegrees F., the corrected speed (for variations in T of the gasgenerator will vary by approximately 25%, when its actual speed is heldconstant. Unless precautions are taken, it is therefore likely that theoutput gearing from the free power turbine may be overstressed when theengine is running in a very cold environment. But if, instead ofimposing a top limit on the actual speed N we control the correctedspeed (N T of the gas generator, the torque Q exerted by the powerturbine becomes independent of the incoming air temperature T as will beshown hereinbelow, where we disclose a simple governor which achievesproper control of the corrected speed.

We now proceed to consider how the fuel flow Wp to the gas generatorturbine T is related to the turbine inlet gas temperature T, and otherengine operating parameters. This analysis is given to facilitate anunderstanding of the problem solved by our invention, and is thereforesomewhat simplified. Thus, we have neglected all secondary effects dueto engine efficiency, heat losses due to radiation, etc., but the basicconclusions are not thereby affected.

The discussion of this section is confined to the gas generator (T C andRC), of FIG. 4, and in Equations l24 hereinbelow C denotes specific heatat constant pressure, B.t.u./lb.R. C, denotes specific heat at constantvolume, B.t.u./lb.-R. g denotes acceleration of gravity 9 H denotesheating value of fuel, B.t.u./ lb. H

J denotes energy conversion factor, 778 ft.lb./B.t.u. 7 denotes ratio ofspecific heats, C /C The turbine inlet gas temperature T; is the sum ofthe temperature T at the exit of the recuperator and the temperaturerise (T -T in the combustion chamber.

Where C and H are as indicated in the example following Equation 5, andW /W is the fuel/ air ratio in the combuster.

To prevent T from exceeding a specified limit, sayT the fuel flow Wadmitted must not exceed When the turbine flow is choked, orsuper-critical, which occurs whenever the pressure ratio across theturbine exceeds about 1.89, the following relation is known to apply.

If this rate of fuel flow is admitted to the engine, the turbine inlettemperature T; will be maintained at its desired maximum, T

For example, assume a gas generator consuming 3 lbs. of air per secondat a turbine inlet temperature of 2400 R. and a compressordischargepressure P =20O'p.s.i.a.

Then

in: X g .735

and if U .24 B.t.u./lb./ F, and H 17,000 -B.t.u./lb. of fuel,

= .000000212 P (T T lb/see. .000763 P (T T lb./hour If the maximumpermissible turbine gas temperature is set at T *=2400 R- then themaximum limiting fuel flow that may be permitted to enter the engine isgiven by the equation and the control system of our invention, asdisclosed herein, produces a relationship such as the above.

The general principle is as follows:

We set a value for T which will be the greatest gas temperature allowedby engine limitations. We then measure the temperature T at therecuperator outlet, and provide means to subtract this measuredtemperature from T We also measure the compressor discharge air pressureP2, and provide means to mechanically multiply this by the quantity justobtained, as described hereinbelow.

The output of this mechanical computer then represents the permissibleprimary maximum fuel flow W to the engine. In the embodiment of ourinvention disclosed hereinbelow, the output is used to vary the positionof a primary fuel metering valve across which a constant pressure dropismaintained,..and thus 'the'fuel flow W is limited in accordance withEquation 5, and the turbine is protected from damage due toovertemperature.

As described hereinafter, while the control is limiting fuel fiow W inaccord with Equation 5, the speed governor is inoperative. But when thecorrected speed is attained for which the manual lever is set, thegovernor will cut in to reduce the fuel flow below the value allowed byEquation 5, in such away that theset speed will be maintained.

Turning now to the free power turbine PT, we shall justify the statementpreviously made, that the torque Q exerted by the power turbine PT isv asingle-valued function of the corrected 'speed of'the gas generatorportion of the engine.

The" hot gas from the gas generator enters the free power turbine PT atstation 5, and expands down to the pressure P prior to flowing. into therecuperator RC, which'is designed so that the gas passing through it hasa very small pressure drop, and the pressure P will be substantiallyequal to the atmosphericpressure.

In this analysis, we again neglect secondary effects such as efficiencyand heat loss, so that the, basic conclusions may be clearlyestablished.

W-henthe exhaust gas fromthe gas generator portion of the engine, attemperature T and pressure P is allowed to expand'freely through anozzle down to pressure P the kinetic energy acquired by each pound ofexhaust gas is given by the expression From (6) we conclude that thevelocity acquired by the gas is u= 2gJc T 1] inches per second If thishigh velocity gas is directed peripherally against a turbine wheel, anddirected through the turbine wheel so that its peripheral component ofmomentum is reversed (as is the case when the high velocity gas impingeson the blades of. a stationary impulse turbine wheel), the torqueexerted on the Wheel is where. r isthe radius at which. the gasimpinges. The torque-Qis then, using (.7)-and (8):

Table l' Corrected speed N/VTI Compressor pressure ratio P /P Corrected"air flow W /vT /P Corrected fuel flow W P /T Corrected turbine inlettemperature T /T From thermodynamics, and the equality of turbine workand compressor work P L 2 "r ilPi) so that T T P s== i .2 1 11 r 1*Hence, T /T is constant, as well as the quantities tabulated in Table Iabove.

Now consider the quantity P /P From adiabatic expansion through theturbine, and assuming no pressure drop from station 2 to station 4, sothat P =P 7-1 11 Ei.Z )=(B) Zi P1 P 2 P1 T4 P1 21 .1 E T T P, So that P/P is constant as well as the quantifies tabulated in Table 1.

Looking again at Equation 9, we now see that everything on the righthand side is constant when the corrected speed is constant, with theexception of P So Q in (8) may be rewritten as a function of any ofthese interdependent constant quantities, like the corrected speed. ThusN 1 (13) P1 f Frequently the compressor inlet pressure is expressed inunits of one standard atmosphere, using the notation and the inlet airtemperature is expressed in units of 518.4 degrees F., using thenotation Using this notation Equation 13 becomes .The above. analysiswas limited to the case of a free power turbine wheel at rest. If thewheel is rotating so that its peripheral velocity is v, then the gasfrom the nozzles approaches the wheel peripherally with the relativevelocity (u-v), and its direction is reversed so that it leaves thewheel with equal relative velocity (uv).

The torque exerted on the wheel is in this case Using a from Equation 7,we have 12 We have already stated in Table I that if (N /VTTF is keptconstant, then (W x/T P is also constant, so that Equation 17 may now bewritten as In Equations 18 and 19, the quantities Q/P and Q/a arereferred to as the corrected torque. These equations are interpreted tomean that when the corrected speeds of both the gas generator and thefree power turbine portions of the engine are kept constant, then thecorrected torque output of the gas turbine is also constant.

If the engine is operating in an environment where the inlet airpressure does not vary appreciably, as in the case of ground vehiclepropulsion, for example, where the engine is nearly always close to sealevel, the quantity 6 is constant, and Equation 19 shows that in thiscase the actual output torque Q is a function of the corrected speeds, N/i/fi of the gas generator and v/Vi of the free power turbine.

Equation 19 shows that maximum torque is exerted by the power turbinewhen it is at standstill-that is, when v=0, and it is in this case thatthe greatest load is applied to the output shaft gearing.

The foregoing analysis shows that for a constant compressor inletpressure P the corrected speed function N/ /T is solely a function ofcompressor inlet temperature T Also, the corrected speed function (NA/Tis a function of the actual output torque (Q) such that there is asingle value of output torque Q for each value of corrected speed N/ /TThe definition of corrected speed NA/T shows that a variation incompressor inlet temperature T, will result in a variation of correctedspeed N/ /T Since it is desired to maintain the actual output torque Qas nearly constant as possible, it is necessary to change the value of Nwhen the value of T changes in order to maintain the corrected speed (N/T constant. The value of N is changed by modifying the fuel flow to thegas generator so that a new value of N is established such that the N//T relationship is maintained unchanged. Thus, the fuel flow to the gasgenerator is regulated as a function of corrected speed which in turn isa function of compressor inlet temperature T It is also plain from (19)that this maximum torque can be limited by placing a maximum limit onthe gas generator corrected speed N A/T; or N {5:

Our invention includes a governor in which the corrected speed of thegas generator may be set by a manual lever and automatically maintainedby a simple governor, and the operation of this device is also disclosedherein.

The foregoing discussion dealt with the speed of the gas generator andwith the fuel flow to the combustion chamber in the gas generatorportion of the engine.

In an actual free power turbine engine, means must be included forlimiting the speed of the free power turbine. In some applications, suchas in ground vehicle propulsion, it may be safe to assume that the powerturbine, directly coupled to the driving wheels, will never overspeedexcept at a dangerously high velocity of the vehicle. In otherapplications, a separate governor, arranged to override the gasgenerator governor, may be included, as disclosed in co-pending patentapplication of Chandler and Wright, Serial Number 494,055, filed March4, 1955 now Patent No, 3,108,435, issued October 29, 1963, and assignedto the same assignee as this application; and does not form part of thepresent invention.

13 PRIMARY FUEL FLOW CONTROL Returning to FIG. 3, it will be noted thatthe fuel and speed control apparatus of our invention, shown in FIG. 2as control unit 13, comprises a fluid-tight case 40, which houses all ofthe operating mechanisms of the control system. Fuel taken from tank 16is conveyed through line 17 and pumped to a comparatively high pressureby pump 42 and then directed to the several control elements containedwithin case 40. Leakage flow from the several control elements containedwithin case 40 provides a flow of suflicient magnitude to fill andpressurize the case 40. The pressure in case 40 is maintained at asubstantially constant control pressure P somewhat less than thedischarge pressure of pump 42, by a springbiased ball check valve 41,through which excess fuel in case 40 is returned (by conduit 18) to fueltank 16. Conduit 17 is connected to the inlet side of a high-pressure,positive-displacement pump 42 (preferably of the gear type), which isdriven by the engine (1), through shafts 43 and 44, connected to shafts26 and 25 (FIG. 2). Fuel discharged by pump 42 flows through conduits 45and 46 to a primary metering valve 47, and thence through a conduit 48,a solenoid shut-off valve 49, a pressurizing check valve 50, and aconduit 51, which connects with conduit 12, leading to the primarycombustion chamber B of the engine.

Conduit 46 is connected by a conduit 52 to a primary metering headregulator 53, which is in turn connected by a conduit 54 to the inletconduit 17 of pump 42, whereby a portion of the fuel discharged by pump42 may be returned to the inlet side of the pump, Whenever the pressurein chamber 55a, acting on a diaphragm 55 in regulator 53, is high enoughto overcome the opposing force of a spring 56, plus the fuel pressure inchamber 57, and open a valve 58. The chamber 57 is connected, by aconduit 59 and a chamber 60, to conduit 48; so that the pressuredifferential acting on diaphragm 55, equals the pressure drop (meteringhead) across valve 47, and said metering head is thus maintained at asubstantially constant value, as determined by the spring 56.

Conduit 45 is also connected to conduit 17, through conduit 61, reliefvalve 62 and conduit 63, so that the fuel pressure in conduit 45 cannever exceed a safe, maximum value, as determined by a spring 64, whichbiases check valve 62 towards closed position.

The metering valve 47 is of the spool type, the upper end of which isattached to a diaphragm 65, between which and a stationary sleeve 66, inwhich the spool valve 47 moves, is a spring 67, so disposed as to urgethe diaphgram upwards. As shown in FIG. 3, the lower side of diaphragm65 is subject to the case fuel pressure P and the upper side of thediaphragm is subjected to a pressure (P n) so that the flow area throughthe metering valve 47 is due to a balance between the upward force ofthe spring and case pressure P and the downward force due to thepressure P acting on the diaphragm.

At the top left of the schematic diagram of FIG. 3 is shown a Ptransducer 68 which'converts the pneumatic discharge pressure (P4), ofthe high-pressure compressor C into a hydraulic pressure (P that is usedto actuate the primary metering valve 47. Fuel at pump pressure (P isadmitted to the lower chamber 69 of the transducer 68 through a conduit70 and restricting orifice 70a. The upper chamber 71 of the transduceris connected by pipe 19 to the discharge passage 20 of the highpressurecompressor, C and the upper and lower chambers are separated by aflexible diaphragm 72 which is biased downwardly by a spring 73.Accordingly, the transducer 68 acts as a pressure regulator,,such thatthe pressure P acting upwards on the diaphragm 72 will balance thedownward force of the spring 73, plus the downward force on thediaphragm due to the compressor discharge pressure P P is the compressordischarge pressure,

A is the area of the flexible diaphragm 72, F is the downward force ofthe spring 73, and P is the regulated pressure then P4+A =PR4 (20)Denote the pressure in the case 40 by P,,. Then subtracting U from bothsides of (20):

( 4 c) R4 e) I At the top of FIG. 3 isshowna T sensor-11, which isresponsive to the temperature T of the air at the exit of therecuperator RC.

The sensor 11 comprises an outer tube 75 of material having a highcoefiicient of expansion, surrounding an inner rod 76 of material havinga low coefficient of expansion. When the sensor is exposed to a streamof gas at a high temperature, the tube, being fixed at its left handend, will expand so that its free right hand end is appreciablydisplaced to the right. The inner rod 76, on the other hand, will remainat substantially its initial length (because of its low expansioncoefficient). The result is that the left hand end of the inner rod 76experiences a displacement to the right when the temperature Tincreases.

The left hand end of the inner rod abuts a pivoted lever 77, with whichis associated an orifice 78a, in such a way that when the inner rod 76is displacedtowards the right, because of an increase in T the orifice78a opens up, and vice versa.

A fuel line, comprising conduits 78, 79 and'80, connects the temperaturesensor 11 to the transducer 68, and consequently the chamber 81 of thesensor 11 is normally filled with fuel from chamber 69 of transducer 68.To prevent this fuel from penetrating into the engine case, a flexiblesealing bellows 82 is provided. The chamber 81 of the sensor 11 isvented to the case 40 of the control by a connecting conduit 43, and isthus subject to the case pressure P The pressure P in chamber 69 of thetransducer 68 is equal to the-compressor discharge pressure P +aconstant C which is determined by preload on spring 73. When orifice 78ais closed by lever 77 (and there is no flow through restrictions'79a and141), the pressure in conduit 79 is equal to pressure P in chamber 69;however, when orifice 78a is opened by lever 77 in response totemperature T fuel flows through restriction 86a, conduit 78, chamber 81and conduit 43 into case 40. This reduces the pressure in conduit 79from P to a lower pressure P which reflects the effect of change intemperature T A restriction 79a in conduit 79 still further lowers thepressure P to P when'there is any fuel flow through branch conduit 84,as hereinafter described. The modified pressure P acting on diaphragm65, in opposition to the case:pressure P and the force of spring 67,actuates the primary metering valve 47 in accordance with the value ofpressure P At the center right of FIG. 3 is shown a speed governor 85,which will be described below. Associated with the speed governor 85isan orifice 84a, closed when the governor is not operating, which is thestate during acceleration of the engine.

We now describe the situation during acceleration of the gas generatorportion (C RC, B and T;,,), of the engine.

The primary metering valve 47, transducer 68, and T sensor 11, withtheir associated elements, determine the primary fuel flow tothe engineduring acceleration, since the orifice 84a'is closed because when thespeed'governor is inoperative, lever 86 closes said'orifice.

' ward force on this diaphragm is (P A The fuel flow to the engine forproper acceleration is, from Equation 5 i Where k is a constant, T isthe desired maximum allowable turbine inlet temperature, and T is themeasured temperature of the air leaving the recuperator.

Fuel from the fuel pump 42 is supplied to the P transducer assemblythrough the orifice 70a, at some pressure P The upper chamber of thetransducer is connected to the compressor discharge chamber of theengine, and is therefore subjected to the pressure P Let P be the fuelpressure in the lower part of the transducer.

Then the downward force on the diaphragm is (P A where A is theefiective area of the diaphragm 72 that separates the upper and lowerchambers 71 and 62 of the transducer, plus the force F of spring 73. Theup- When these forces are in equilibrium, we have already shown(Equation 20) that and the valve 72a attached to the diaphragm 72 willopen 'or close to pass more or less fuel to the case of the "control atpressure P to maintain the pressure P at 80a and 78a is denoted by P inFIG. 3. From the law of pressure drop through orifices PRi'P., PRr-P)where A and A below denote the areas of orifices 78a and 80arespectively, and substituting (21) in (22) The case pressure P may beregulated to any value we please; in particular, we may make P =F /A andin that case & AD

' may be made any other value necessary to produce the desired engineacceleration schedule by biasing P above P Primary metering valve 47receives fuel from the main pump 42 at pressure P and delivers it to theengine at pressure P The pressure difierence (Pg-P is maintained at aconstant value by the by-pass pressure regulating valve 53.

It will be seen in FIG. 3 that diaphragm 65 to which is attached thespool of the metering valve 47 is urged downward by the pressuredifierence (P P when there is no fuel flow through orifice 7&1, and isurged upward by spring 67 whose rate is ks.

The downward displacement of the metering valve is thereforeproportional to the pressure (P P and if the port a e? of the metefitugvalve 47 is proportional to its downward displacement, then the flow offuel through the valve is also proportional to (F -P i.e.,

r' R4'-" c) or substituting Equation 24 seen that the control shownschematically in FIG. 3 will produce the correct fuel flow provided Theorifice a of FIG. 3 being assumed fixed, it is readily apparent that asT increases, the opening of orifice 78:: increases, and the quantity onthe left hand side of (26) decreases. The manner in which this quantitydecreases with increasing A in a somewhat non-linear way, is shown inFIG. 6.

At the right end, the floating lever 86 is subject 'to the upward forcefrom a spring 89 that tends to rotate the floating levercounter-clockwise about the movable fulcrum. The lower end of thisspring is urged upward by a bellows as shown. The bellows 9t), and atemperature-responsive bulb 1, which is connected to bellows 90 by aconduit 92, are filled with an incompressible fluid, and exposed to thecompressor inlet air stream. The upward expansion of the bellows 9t),and therewith the upward displacement of the bottom of the spring 89, istherefore proportional to the air stream temperature 1) When the upwardforce of fiyweights 87 is low enough, the spring 89 and temperaturebellows 90 will rotate the lever 86 about the movable fulcrum 88 untilthe lever bottoms on the orifice 34a, closing it effectively againstleakage of fuel, and when this is the case, the upward force exerted bythe spring 89 is proportional to the temperature T say k T The distancebetween the point of application of the upward force of flyweights 87and the spring 89 being I, and the movable fulcrum being positioned bythe manual lever at a distance x from the spring 89, we observe that theforce k N exerted by the flyweights 87 will overcome the force k Texerted by the temperature bellows 90 and spring 89, whenever k N (lx) kT x or when N lo a: ii an or when That is, whenever the corrected speed"(HA/T of the engine exceeds some value corresponding to a manuallyselected magnitude of x, the flyweight force will overcome thetemperature spring force, and rotate the floating lever 86 in theclockwise direction. This causes the lever to separate from its restingplace on orifice 84a, and the orifice will open.

From FIG. 3, it will be seen that the opening orifice 84a will cause thepressure P to drop to a lower value P because of the pressure dropacross orifice 79a, and the result of this is a closing down of the mainmetering valve 47 with a consequent reduction in fuel flow W to theengine. This reduction in fuel flow will prevent the engine fromcontinuing its previous acceleration, and the speed of the engine willbe held constant at the selected corrected value.

The tube 75 of the T temperature sensor 11 is im- 17 mersed in the gasstream at the exit from the recuperator (station 6 in FIG. 1).

We stated previously that the sensor 11 consists of an outer tube 75 ofhighly expandible metal, the tube being fixed at its left hand extremityto the case 74 and incloses a rod 76 of low coefiicient of temperatureexpansion, the rod being attached to the tube 75 at its right hand end.

Now fixing attention on the left hand end of rod 76, it will be plainthat this point will be displaced towards the right as the temperatureof the whole assembly is raised, and vice versa, due to the relativedifferential expansion of the tube 75 and the rod 76, so that anincrease in the temperature T results in a proportionate opening of theorifice 78a.

We mentioned above the anticipating effect of the T sensor. Suppose thetemperature T suffers a sudden change (say an increase) from apreviously existing steady value. This increase will be immediatelydetected by the highly expansive outer tube 75, the free end of whichwill at once begin to move to the right, pulling the inner rod 76 withit and opening the orifice 78a. Some appreciable time later, heat due tothe increase in T will penetrate into the low coelficient of expansioninner rod 76, which will then move in a direction tending to close theorifice 78a. By suitable matching of the time delays in the temperatureresponse of the two elements 75 and 7 6 of the sensor 11, the netover-all response can be matched to the response of the recuperator RCitself, so that transient effects of changes in T can be compensatedfor.

The operators control of the speed and power of the engine 1 is effectedby a manual control lever 93, which is fixed to a shaft 94, journalledin the case 46 and extending into control unit 13, as shown in the upperright corner of FIG. 3. Also fixed on shaft 94 is a cam 95,

on which rides a follower whose other end is attached 'to a piston 97,so that when cam 95 is rotated through an angle PLA which increases itspitch radius, piston 97 is moved to the left in a cylinder 8, againstthe force of an opposing spring 99. A rod 100 connects piston 97 withroller 88 and piston 97 is provided with a passage 10 1 through whichliquid may pass only from left to right by virtue of check valve 191a.Motion of piston 97 is not therefore effected to the left, howevermotion to the right is at a retarded rate determined by the size of theopening of passage H52, as determined by adjusting needle valve 193 intocase pressure P so that cylinder 98 and piston 97 function as adash-pot, to'retard the rate at which a movement of lever 93 may movethe roller 83 to the right only, and thereby change the rate ofretarding the N throttle setting of the engine. The retarding dashvpotaction of cylinder 93 and piston 97 is required to prevent such suddenchanges in fuel flow W (upon .quick movements on lever 97) as may causecompressor and engine stall or burner blow-out.

Liquid :fuel is displaced from cylinder 98 through a passage 1532 whichcommunicates with the interior of case 40, and such admission of fuel iscontrolled by an adjustable needle valve 1433, which is screw-threadedthrough the wall of case 40 and provided with a slot 104, whereby thevalve 193 can be adjusted from the outside of case 49.

Solenoid valve 49 is adapted to open and close communication betweenconduit 48 and chamber d4), upon energizing or deenergizing of asolenoid 195, which is accomplished by manually closing or opening aswitch 196, in an electric circuit 197 to which current is supplied by abattery 1% (as shown in FIG, 2). When the operator desires to stop theengine 1, he opens switch 166, which deenergizes solenoid 1G5, andcloses valve 49, whereupon the fuel flow W to the engine is shut off.

The ball check valve 59 maintains a desired fuel pressure in the fuelcontrol apparatus at all times, and prevents escape of fuel from chamber60, when valve 49 is closed.

SECONDARY FUEL FLOW CONTROL So far, we have considered those elements ofour fuel control apparatus, depicted in the upper part of FIG. 3, thatregulate the primary fuel flow to the engine. We will now describe theelements in the lower part of FIG. 3 which control the secondary fuelflow to the engine.

A conduit 1-10 connects conduit 61 with branch conduits 111 and 112.Fuel from 111D flows through a restriction 1&3 in conduit 112, under areduced pressure P into the lower chamber "114 of a transducer 115, andfrom thence through a restriction 1 16 in aconduit 1-17, under a reducedpressure P into a chamber 118 of a secondary fuel metering valve 119. Aflexible diaphragm 120 separates chamber 118 from a chamber 121, intowhich fuel enters from case 40 through a port 122 at case pressure P sothat said diaphragm is subject to the pressure differential (P Diaphragm120 is attached to valve 119 and a spring 123 biases said dia phragm tothe left, whereby the fuel flow area through valve 119 is varied inaccordance with the fuel pressure differential (P 1-P and the rate ofspring 123.

Fuel flows through conduits 110 and 111, a chamber 124 (of a throttlingmetering head regulator 125), a valve 126, a chamber 127, conduits 12$,r129, 13% and 131, a solenoid shut-off valve 132, a check valve 133, andcon duit 14 (of FIG. 2) to the secondary combustion chamber B of theengine 1.

In the metering head regulator 125, a diaphragm 134 separates chamber127 from a chamber 135, connected by conduits 131: and 137 to conduit131, and houses a spring 138 that biases valve 126 (attached todiaphragm 134) towards the open position. Valve 126 reduces the fuelpressure from a value P in chamber 124, to a value P in conduits 128 and1129, and valve 119 reduces the pressure P to a value P in conduits and131. Conattached to a flexible diaphragm 145, separating chamber I 142from a chamber 14-6 that is connected by a conduit 147 with conduit 84.A spring 148, attached to diaphragm 14-5 biases valve 144 towards closedposition, in opposition to the fuel pressure differential betweenchambers 142. and 146 acting on diaphragm 145. The flow area throughvalve 144 establishes an additional fuel ilow path from conduit 12-? toconduit 137, which augments the fuel flow through secondary meteringvalve 1-19.

A conduit 15% connects conduit 1 17 (upstream of restriction 116) with achamber 151 which is closed by a flexible diaphragm 152, having anattached fixed pivot 15 that contacts a floating lever 154. The rightend of lever 154 is biased upwardly by a spring 155, so as to rock in acounterclockwise direction about a movable roller pivot 156, inopposition to the upward thrust of fixed pivot 153 by diaphragm 152. Themovement of the left end portion of lever 1 54 varies the flow areathrough nozzles 157 and 153 at the upper ends of conduits 159 and use.Conduit 159 is connected to conduit by a conduit 161, so that variationsin the fiow area of nozzle 157, by movement of lever 154, cause-svariations in the pressure in conduit 14% and chamber 142.

Roller pivot 15% is connected by a link 162 to a roller cam follower 163which rides upon a'cam 164 to which it is held in contact by a spring165. Cam 164 is pivoted at 166 and is connected by a link 1&7 with cam95, so that as cam 515 is rotated. by lever 93, earn 164 is alsocorrespondingly rotated, about its pivot 166. Spring seats in a movableabutment 168, supported by the left end of a lever 169, which is pivotealt-170, and whose T9 right end is interposed between spring 8b andbellows 0, so that expansion or contraction of said bellows (re sponsiveto air inlet temperature T in tube 91) varies the tension of spring 155and the loading of lever 154.

The lower end of conduit lid has a restriction 171, and is connected bya conduit 172 to a chamber TF3, which is closed by a flexible diaphragm174. A link 175 connects diaphragm 174 with a lever 376, which ispivoted at 177 and biased in a clockwise direction by a spring 178.Movement of the right end portion of lever 175 varies the flow areasthrough nozzles 179, 18% and 181, into a chamber 182 which communicatesthrough a port 183 with case 4%, whereby the pressure in chamber 182 iscase pressure P A conduit 184 connects conduits 117 and 166, so that thepressure (P in 117 is varied by variations in the flow area throughnozzles 158 and 179. Nozzle 253i? is connected by a conduit 185 withconduit lei, so that the pressure in chamber 142 is varied by variationsin the flow areas through nozzles and 18d. Nozzle 1.82 is connected by aconduit 186 with chamber 1% pressure from conduit 147 may be dischargedthrough conduit 186 and nozzle 18!, so that the pressure in chamber 14sis varied by variations in the flow area through nozzle lltil.

engine 1 to the fuel control unit 13 by conduit 23, which is connectedto a chamber 1%, and said pressure and a spring 1% act on a diaphragm92, attached to a valve 393,

which controls the fuel fiow from chamber 114 into case 40, wherebytransducer 115 converts the pressure P to a control pressure P 'Conduit1'72 connects conduit 11% (downstream of restriction 1'71) with anannular groove 21a in a power turbine speed governor 211, which isdriven by the engine l, through connected shafts 29, 28, 27 and 15 (PEG.2). Drive shaft 29 is connected to a spindle 212 which terminates at itsupper end in a horizontal cylinder 2R3, having at its opposite ends casepressure port 214 and outlet port 21d. Cylinder 213 rotates in a chamber215 which communicates through a port 217 with case 4%. A passageway 218and 21% connects conduit 172 with the interior of cylinder 213, in whichis mounted a slidable spool valve 219 that is biased toward port 215 bya spring 22%. Valve 219 has a passage 221 through which liquid fuelflowing through conduit 172 and passageway 218 escapes into the rightend of cylinder 213 and from thence through port 215 into, chamber areto the case via opening 27.

By virtue of the foregoing arrangement of the elements of speed governorZlll, said governor generates in conduit 172 a pressure (P which isproportional to the speed (N of the power turbine PT. A conduit 222connects conduit 172 with a speed (r.p.m.) guage 223 which is located sothat the engine operator may know the speed (r.p.m.) of the powerturbine at all times. T

Ignition fuel fiow required to start the engine 1 is provided through aconduit 187, having a manually adjustable needle valve 188, which isconnected to primary fuel conduit 12, whereby fuel is fed intocombustion chamber B OPERATION OF CONTROL SYSTEM The fuel control systemof our invention determines two fuel flows to the engine, viz.: aprimary fuel flow and a secondary fuel flow, as follows:

l where W primary fuel flow P =high-pressure compressor dischargepressure T =recuperator temperature A A A =design constants and W=secondary fuel flow P =l0W-pressure compressor discharge pressure A A=design constants W ,'=secondary augmentation flow and is proportionalto the amount of cut in of the primary fuel flow governor (the Ngovernor).

Both primary and secondary fuel flows W and W are scheduled as afunction of the positions of a primary metering valve 47, and asecondary metering valve 119, in a system where the metering heads, orpressure drops across these valves are held constant.

The primary metering head regulator 53 bypasses fuel flow around thefuel pump &2 and the secondary metering head regulator throttles thefuel flow, as required to hold a constant drop across the meteringvalves of both circuits. The magnitude of metering head is determined bythe area of the diaphragm in each regulator, divided into the preload ofits spring.

Identical pressure transducers 6S and M5 are used in the primary andsecondary fuel circuits to convert pneumatic engine signal pressures tohydraulic pressures. The pressure transducers throttle incoming pumpdischarge pressure to scheduled levels above the pneumatic P and Pengine pressures. These scheduled pressures being P and P respectively,as expressed by:

The values of C and C are determined by the spring preload on thetransducer diaphragm.

The variable bleed circuits in both the primary and secondary fuelsystems serve to modulate the transduccd pressures P and l foracceleration fuel scheduling. These circuits are as shown in the upperand lower parts of FIG. 3, respectively.

During acceleration, bleeds 34a, 157, 153 are closed, as well as 179,and 181 which open only during power turbine governor operation. Thepower turbine PT acts only as a topping governor to prevent overspeed ofthe output shaft 15. Therefore, the modulated pressure P and P establishthe respective primary and secondary acceleration flows as follows:

With orifices closed as noted above, PR4'ZPR4H, and for a fixed value ofT acceleration flow would be proportional to P +a constant. However, bybleeding off through variation in T which varies orifice 78a, theschedule can be proportionally changed as a function of (Ag-T5) to meetthe schedule dictated by the equation (A) above.

With orifices closed, as noted above, P :P and the schedule noted abovecan be achieved, since and since metering head is constant and fuel flowis a function of P As the (N speed governor 85 cuts in, a pressure dropoccurs across orifice 79a, with the commencing of flow through orifice84a. This pressure drop is a function of the opening of orifice 84a,which determines the flow and hence, this drop (P P across 79a. Thispressure drop across the augmentation valve 343 will produce a stroke ofthis valve proportional to (P P since stroke is determined by the areaof valve diaphragm 14S, and the preload and rate of spring MS.Increasing the governor 85 cut-in increases (PR4'P"), which producesincreasing stroke and hence, increasing fuel flow. The secondary systemfuel metering head is maintained across the augmentation valve 144,hence making change in fuel flow a function of stroke.

21 Maximum augmentation fuel flow can be limited by a maximum flow stopon this valve.

Therefore W is defined as follows:

where N (N governor 85 cut in speed N (N actual speed after cut-in.

Primary and secondary steady state fuel flows, W and W are achieved bymodulating P and P pressures. In the primary circuit, P pressure ismodulated by the N governor 85, via bleed 84a. In the secondary circuit,P pressure is modulated by the P governor, via bleed 158. Primary andsecondary fuel flows are thereby established in accordance with theerror in set (N speed and (P pressure parameters.

Governor droop rates may be varied by changing bleed 79a in the primarycircuit, and by changing bleed 116 in the secondary circuit. Reducingthe flow areas for each bleed will respectively increase droop rates.This adjustment may also be made by changing the rate of spring 89 ofgovernor 85, or the rate of spring 155 of the P governor.

The change in T temperature, as sensed by the T sensor 91, produces achange in length of the T motor bellows 90, which varies the set preloadon spring 89 of the governor 85. Increasing T temperature increases thebellows 90 length, which in turn increases the preload of the governorspring 89, and reduces the preload of the P governor spring, to correctset speed conditions, as a function of T temperature, in order to meetthe T correction requirements.

The selection of pressure (P and speed (N as a function of throttleangle PLA, is determined by the N and P governor cams 95 and 164respectively, which set the lever arm ratio between spring preload andset reference forces on the respective governors.

A spring loaded dash pot, 97-99, with provision for a variable flowrate, limits the deceleration time of the high-pressure compressor (Cspool. In the increasing speed direction, the dash pot 97-99 has noeffect and, therefore, a set speed is established directly by powerlever angle PLA. A screw adjustment 103 is provided to vary the area ofport 102 which bleeds flow from the dash pot during deceleration of theN spool (C2Tb This area may be adjusted to permit decelerations of theorder of less than 1.0 second to more than 15 seconds.

The delaying of (N deceleration prevents the lowpressure compressor Cfrom going into surge, by equalizing the deceleration rates of the tworotor spools, (C -T and (C T The (C -T spool has a much lower inertiahence, tends to decelerate faster than the (C -T spool.

The power lever 93. may be set in any position from idle to maximumpower before the engine is cranked. Minimum primary acceleration fuelflow is supplemented with the preselected ignition flow for properignition. Secondary flow is shut off by solenoid-operated valve 132 inthe secondary fuel line until primary ignition occurs. After primaryignition, secondary fuel flow is turned on, providing acceleration flowfor secondary ignition. As the engine accelerates, acceleration flow ismetered in both fuel circuits until the preselected steady state speed(N or pressure (P is approached. Since the N spool accelerates muchfaster than the P pressure spool,

'the N governor will cut in first, thereby opening bleed 22 P pressureis approached, the P governor cuts in, thereby opening bleed 158, andcausing secondary fuel flow (W to be reduced in accordance with thecontroldetermined droop gain (W /P error). Secondary fuel flow (W willcontinue to be reduced after governor cut-in as the P pressureincreases, until the required secondary steady state fuel flow isachieved. The P pressure will then be controlled at a predeterminedpressure above P governor cut-in. The supplementary ignition flow willbe turned off at a predetermined N speed or F pressure. Subsequentincreasing of the power lever angle will increase N speed and P pressureto prescheduled levels as a function of power lever angle.

Engine deceleration is achieved by retarding the rate of change in thepower lever angle PLA. A direct reduction of secondary fuel flow (W willresult in a flow level which is set, either as a function of thesecondary circuit bleed sizes, or to the minimum flow established by anadjustable flow stop, whichever limits the higher level. Although the Ngovernor lever has been retarded, which would normally result in adirect flow reduction as in the secondary system, dash pot 97-99 limitsthe retardation of the lever cam follower 96 of the N governor. N speedis reduced, therefore, as a function of time. This action prevents anundesirable mismatch in speed of the two engine spools duringdeceleration. Fuel flows in both circuits is reduced until the newdesired steady state parameters are achieved.

Normal engine shut down may be effected by retarding the power lever 93to the idle speed position, and then shutting off fuel flow in both fuelcircuits by operation of solenoid valves 49 and 132.

If, during operation of the engine, the power turbine speed (N exceeds apreset value, as determined by the spring preload setting on the powerturbine governor diaphragm 151, the power turbine governor will operateto open orifices 179, 189 and 181, which in effect cause cut-in of the Pand N governor circuits, and bleed off of the hi h-pressure side of theaugmentation valve diaphragm 145, to effect closing down of the threemetering valves 4-7, 119 and 144 in the control. The closing down ofthese valves is proportional to power turbine governor 211 set speederror, and the respective gains are determined by the combinations ofsizes of orifices used in the respective circuits.

A pressure signal is generated by the power turbine governor system toprovide a guage 223 indication of power turbine speed. This pressure mayalso, if desired, be used to operate a preset pressure switch whichwould deenergize the solenoids and shut down the engine.

Ignition fuel flow to start the engine is provided from conduit 111)through an adjustable needle valve 188 in branch conduit 187, of theengine primary fuel circuit; and an engine solenoid actuated valve 189is used to initiate and turn off ignition flow.

The high-pressure spool (C -T is cranked by a starting motor (notshown), the primary ignition switched on, and solenoid valve 49 isopened by closing switch 1%, to start primary fuel flow W which issupplemented by ignition fuel flow through valve 138. The primary flow Wwill be metered in accordance with T temperature and 1, pressure, asspecified by the primary acceleration schedule (FIG. 7). The ignitionfiow will be provided by a parallel path through the adjustable fuelflow valve 188, and solenoid valve 189, in the control unit 13; and thisignition flow is added to the control primary fuel flow. The amount ofignition flow may be trimmed as required to individual enginerequirements.

When ignition occurs and the temperature sensor 11 senses thetemperature in the primary combustion in chamber B switch 190 (FIG. 2)is closed. This will shut off ignition flow, and energize the secondaryfuel flow solenoid valve 132. The fuel flow, scheduled with low-pressurecompressor delivery pressure (P (FIG. 7),

will determine the star-ting flow to the secondary combustor B Uponignition, the engine will accelerate to slow idle speed, it the powerlever has not been advanced to any other position that may be requiredby an arbitrarily selected power lever setting. The engine isaccelerated by advancing the power lever 93 from any power lever settingto a higher power lever setting. The primary system accelerationschedule is defined by P pressure, modified as a function of recuperatortemperature T as shown in FIG. 8.

Engine compressor pressure P is sensed by the control P hydraulictransducer 68, which produces the equivalent hydraulic pressure P P ismaintained at a constant value above R, by the selected preload onspring 73, to produce the required schedule as defined in FIGURE 7, andallows the control to reduce fuel flow below the starting value duringgoverning. The P transducer 73 operates on pump pressure P as suppliedthrough orifice '7Ja and bypassed through regulated orifice 68a.

P is supplied to the W metering valve diaphragm dthrough orifices Miaand 79a. The pressure between these two orifices (P is varied by the T 6temperature sensor 11 which in effect reduces P to a value (P lower thanP by the amount of increase in T temperature or bleed oil throughorifice 73a. The T sensor 11 operates on difierential expansion betweenthe tube 75 and contained rod 76, to uncover orifice 78a as a functionof T With orifice 34a closed, and no flow through orifice 141, thepressure P (equal to P positions the W metering valve 47 against itsloading spring 67.

Fuel flow W through the metering valve 47 is a func tion of valveposition only, since the primary flow metering head regulator 53,operates to maintain a constant pressure drop across the metering valve47. The metering head regulator 53 senses'this differential pressure andbypasses pump output flow to pump inlet 17, as required to regulate themetering head.

The control pressure P is sensed by transducer 125 and converted to therequired biased P pressure, by the same method that was utilized by theP transducer 68.

The pressure P equal to P (when there is no flow through orifice 116),positions the W 1 metering valve 114- in accordance with therequirements of the engine, as shown in FIGURE 7. The position of themetering valve 119, as with valve 47 in the primary system, determinesthe secondary fuel flow, (W since the pressure differential across valve119 is maintained constant by the secondary metering head regulator 125.

The secondary flow metering head regulator 125 senses the pressuredifferential across metering valve 119 and throttles the pump outputpressure P which is established by the primary flow system, to maintaina fixed head across the secondary metering valve 119 (and, as will bediscussed later, the augmentation flow valve 144) The fuel controlsystem will accelerate the engine until steady state conditions (asdictated by the power lever setting) are achieved. Steady state controlis maintained by the N governor 85 and P governor, respectively, for theprimary and secondary systems. General governor operation is indicatedin FIGURE 9 for both the N and P system.

The N governor 85 setting is established by the preload force in thespring 89 loading the governor flyweight 37 toes. The preload of thespring 89 is increased with increasing power lever angle PLA through thecam 95, roller 88, and lever 86 arrangement to increase set speed withpower lever angle. The T temperature sensor also increases this preload,with increasing T temperature, to maintain set values of correctedspeed. The governor S5 setting, as a function of T and PLA, is indicatedby FIGURE 10.

PLA), the governor d5 cut-in begins. The flyweight 37 toes lift lever 86and uncover orifice 184a, reducing P and hence, W The rate of cut-inwith speed error is determined by the net governor spring 59 rate,flyweight 87 design, and the selected orifices of the primary me eringcomputing system. During governor cut-in, a pressure drop, as a functionof the amount of cut-in, will appear across orifice 79a. This pressuredrop is used to position the augmentation flow valve 144 in thesecondary flow system.

The P governor cut-in point is determined by the sp F preload, actingagainst the diaphragm 152. The opposing force is set, in the increasingdirection with increasing power lever angle FLA, by the P governor cam16 2. This opposing force is also biased to decrease, with increasing Ttemperature, through a linkage 169 from the T sensor bellows 39', tomaintain a value of corrected N setting at any selected value of FLA.

As N speed increases, increasing the corresponding P and P pressures,the diaphragm 15?.- force will overcome the opposing preload from spring155, and uncover orifice 153, effecting cut-in of the P governor bybleeding off pressure The rate of cut-in, with pressure error signal, isa function of the diaphragm 152. size, net governor spring 15% rate, andthe selected orifices of the secondary metering system.

The pressure drop across orifice We, applied across the diaphragm T45,acting on the augmentation valve Md, will add fuel flow to the secondarysystem in parallel with the secondary metering valve 114, proportionalto the amount of N governor cut-in. This augmentation fuel flow N willbe zero at zero cut-in and rise to a maximum preset value at somepercentage of cut-in.

Since the augmentation flow W is desired only during acceleration of theengine, means is provided to shut off augmentation flow duringdeceleration. This will be discussed below. With decreasing N shafthorsepower load, augmentation flow to the secondary is increased tooptimize performance.

The engine is decelerated by retarding the power lever 93 from anyhigher power lever setting to a lower lever setting.

Retarding the power lever angle will unload the governor fiyweignts 37,causing the tees to rise to a high position and completely uncoverorifice 84a. This reduces P pressure to a lower value, causing themetering valve 27 to close off toward a minimum value, and reducing theprimary metered flow N to the engine. The engine will then decelerate toa new steady state operating point, corresponding to the newly selectedpower lever angle.

The N governor dashpot 97-99 will retard, or cause a time delay, in therate of reset of the N speed, in the decelerating direction, to equalizethe deceleration times of the two engine rotors and prevent low-pressurecompressor surge, as indicated in FIGURE 11.

Retarding of the throttle 93 will, (as in the primary system) uncoverbleed 15$, reducing pressure P and hence secondary fuel flow W The Nspeed will reset at a lower value, corresponding to the new lowerselected throttle position, as dictated by the P pressure.

As previously mentioned, the augmentation fiow is not desired duringengine deceleration because deceleration time will be increased. Inaccordance with this requirement, reduction of the throttle angle FLAwill produce simultaneous opening of orifice 157, causing the highpressure signal to the augmentation valve 144 to be reduced (noteorifice lldl in the line 14d to the augmentation valve). Theaugmentation valve 1- 34 will then close during decelerations, remainclosed during accelerations, and open only during N governor cut-in.

The power turbine speed governor 21 operates to cutin and reduce N and Psettings sinultaneously at the instant that power tur ine set speed isexceeded. The centrifugal force generated by the spinning valve 219 isbalanced by the pressure P developed between orifices 171 and 173, Thepressure P acting on the diameter of the valve 219 in the control will,at the overspeed condition, cause an unbalance condition between thediaphragm spring 178 load and the pressure generated diaphragm 174force. This unbalance will simultaneously uncover orifices 178, 18th and181 to reduce primary and secondary metered fuel flows, and shut off theaugmentation fuel flow. The relative rates of cutback of the primary andsecondary fuel flows will be determined by selection of the diaphragmloading spring 178, diaphragm 17d size, and relative sizes of orifices179, and 181. The loading spring 220 on the rotating valve 219 in thepower turbine control has been included to insure that no leakage occursduring the cranking and starting of the engine.

The engine will be shutdown by turning off the ignition switch on valve189 to simultaneously close both primary and secondary solenoid valves49 and 132 and stop all fuel flow to. the engine.

Both circuit pressurizing and check valves 59 and 133, which are mountedas close as possible to the engine nozzles, will prevent undue dribblingof fuel into the combustion chambers B and B and minimize coking of thenozzles therein. This might otherwise be a problem caused by long fuellines slowly emptying into hot combustion chambers.

The'control system case pressure (P is maintained at essentially ambientpressure by connecting case 4% to the fuel tank through a check valve41. With the control case pressure P, fixed at an ambient reference, allhydraulic pressures computed in the control refer to this fixedreference, eliminating the need for balancing diaphragms. i

The requirement of control operation submerged in deep water, such asmay be called for in a tank type vehicle, will present no problem, sinceall ambient references, such as exposed bellows and/ or diaphragms, havebeen eliminated. The only required ambient reference of the control willbe a single line back to the fuel tank.

While we have shown and described the preferred embodiznent of ourinvention, we desire it to be understood that we do not limit theinvention to the precise construction and arrangement of elementsdisclosed by way of illustration, since these may be changed andmodified by those skilled in the art without departing from the spiritof our invention or exceeding the scope of the appended claims.

Vvle claim:

1. In operative association with a gas turbine engine, having acompressor supplying compressed air through a heat recuperator to a fuelcombustion chamber, a fuel control comprising: means supplying fuel tosaid combustion chamber, at a rate varying in accordance with the massrate of air flow thereto, and means for automatically modifying the rateof said fuel supply, in accordance with a function of the product of thecompressor discharge ressure and the temperature of the air leaving saidrecuperator, so that the temperature of the gases leaving saidcombustion chamber never exceeds a preselected maximum limit.

2. In operative association with a gas turbine engine, having acompressor supplying compressed air through a heat recuperator to a fuelcombustion chamber, means for discharging combustion gases generated insaid chamber successively through said turbine and recuperator; a fuelcontrol comprising: means supplying fuel to said combustion chamber, inaccordance with the mass rate of air flow therethrough, and means forautomatically modifying the rate of said fuel supply in accordance withthe temperature of the air leaving said recuperator so as to compensatefor the heat added to said air by said recuperator, whereby thetemperature of the gases discharged from said combustion chamber intosaid turbine never exceeds a preselected maximum limit, including meansfor limiting the maximum fuel flow to said combustion chamber inaccordance with the equation:

wherein (W 1 is the rate of said fuel flow, (K) is a preselectedconstant, (P2) is the discharge pressure of said compressor, (T,;*) isthe preselected maximum permissible temperature of gases entering saidturbine, and (T is the temperature of the air leaving said recuperator.

3. A fuel control according to claim 2, having means for measuring therecuperator outlet air temperature (T means for automaticallysubstracting said temperature (T from said maximum temperature Tfi,means for automatically measuring said compressor discharge pressure (Pand computer means for mechanically multiplying the dilference betweensaid temperatures by said measured pressure (P 4. A fuel controlaccording to claim 3, having a positionable fuel metering valve forregulating the fuel flow (W 1 means for maintaining a constant pressuredrop across said valve, means to vary the position of said valve, inaccordance with the output of said computer means, and means to limitsaid fuel flow, in accordance with said equation, so that said turbineis protected from damage due to overtemperature.

5. A fuel control according to claim 4, wherein said engine comprises afirst and a second gas turbine, said first turbine drives saidcompressor, and said second turbine is a free power turbine whichdelivers the power output of said engine; and means for passing thegases discharged by said first turbine successively through said secondturbine and said recuperator, so that the heat of the gases passingthrough said recuperator is utilized to heat the compressed air passingthrough said recuperator.

6. A fuel control according to claim 5, having a positionable manualpower control lever, and a speed gover nor driven by said first turbine,for regulating the speed (N of said first turbine, means for modifyingthe action of said governor in accordance with the temperature (T of theair entering said compressor and the angular position of said manualcontrol lever; means for rendering said governor inoperative when thecontrol is limiting said fuel flow (W in accordance with said equation;and means for cutting in said governor when the corrected speed (N /x/Tof said first turbine attains a value for which said manual lever isset, so that said set speedis maintained under varying operatingconditions.

7. A control according to claim 6, having means for controlling thecorrected speed (N /T of said first turbine, so that the torque (Q)exerted by said second turbine is a single-valued function of saidcorrected speed, whereby the maximum value of said torque (Q) is limitedby a selected maximum value of said corrected speed.

8. In operative association with a gas turbine engine, comprising: afirst, low-pressure air compressor driven by a first gas turbine, asecond, high-pressure air compressor, divert by a second gas turbine,for supplying compressed air through a heat recuperator to a first fuelcombustion chamber, a third, free-power turbine, interposed in gas' flowseries relation between said first and second turbines, for generatingthe power output of said engine, and a second fuel combustion chamber,interposed in gas flow series relation between said second andthirdturbines; a single-unit fuel control comprising: means for coordinatelyregulating fuel flow from a common supply source to said first andsecond combustion chambers, in accordance with the rate of mass air flowthrough said engine, and means for automatically modifying the fuel flowto said first combustion chamber, in accordance with the temperature ofthe air leaving said recuperator, so that the temperature of the gasesleaving said first combustion chamber never exceeds a preselectedmaximum limit, in

3,1 ease-e c; v a? eluding means for limiting the maximum fuel flow tosaid first combustion chamber in accordance with the equation:

wherein (W is the rate of said fuel flow, (K) is a preselected constant,(P is the discharge pressure of said second compressor, (T,,*) is thepreselected maximum permissible temperature of gases entering saidsecond gas turbine, and (T is the temperature of the air leaving saidrecuperator.

9. A control according to claim 8, including means, re-- sponsive to thetemperature (T of the air entering said first compressor,'forautomatically correcting said fuel flow to the first combustion chamberfor each particular (T temperature condition, so as to provide thedesired steady-state operation of the engine, at optimum fuel consumption rates and optimum temperature condition in said recuperator,without encounting compressor surge and turbine overternperature.

10. A fuel control according to claim 9, having means for measuring therecuperator outlet air temperature (T means for automaticallysubstracting said temperature (T from said maximum temperature T5 meansfor automatically measuring said compressor discharge pressure (P andcomputer means for mechanically multiplying the difference between saidtemperatures,

by said measured pressure (P 11. A fuel control according to claim 10,having a positionable fuel metering valve for regulating the fuel flow(W to said first combustion chamber, means for maintaining a constantpressure drop across said valve, means to vary the position of saidvalve in accordance with the output of said computer means, and means tolimit Said fuel flow, in accordance with said equation, so that said gasturbine is protected from damage due to overternperature.

12. A fuel control according to claim 11, having a positionable, manualpower control lever, and a speed governor driven by said second turbine,for regulating the speed (N of said second turbine, means for modifyingthe action of said governor in accordance with the temperature (T of theair entering said first compressor and the angular position of saidmanual control lever; means for rendering said governor inoperative whenthe control is limting said fuel flow (W 1 in accordance with saidequation; and means for cutting in said governor when the correctedspeed (N /Vi) of said second turbine attains a value for which saidmanual lever is set, so that said set speed is maintained under varyingoperating conditions.

13. A control according to claim 12, having means for controlling thecorrected speed (N /V11) of said second turbine so that thetorque (Q)exerted by said free power turbine is a single-valued function of saidcorrected speed,

whereby the maximum value of said torque (Q) is limited by a selectedmaximum value of said corrected speed.

14. in operative association with a gas turbine engine, comprising: afirst, low-pressure air compressor, driven by a first gas turbine; asecond, high-pressure air compressor, driven by a second gas turbine;for supplying compressed air through a heat recuperator to a first fuelcombustion chamber; and a second fuel combustion chamber, interposed ingas flow series relation between said first and second turbines; asingle-unit fuel control, comprising: a manual power control lever,means for coordinately regu lating fuel flow from a common supply sourceto said first and second combustion chambers, so as to maintain theratio between the corrected speeds of said first and second compressorrotors at a selected definite value, for each particular position ofsaid lever, so that for each lever position a selected value of secondcompressor rotor speed, and a corresponding steady-state dischargepressure of said first com ressor is obtained, under varvin operatingconditions; the correcte' speed of each of said rotors being defined asits actual speed (N), divided by the square root of the temperature (Tof the air entering its compressor,

ii. A fuel control as in claim 14, wherein said control includes meansfor modifying fuel flow to said first combustion chamber, in accordancewith the temperature of the air leaving said recuperator, so as toachieve optimum fuel consumption rates, during engine steady-stateoperation, Without encountering compressor surge and turovertemperature.

16. A fuel control as in claim 15, wherein said engine also comprises afree power turbine for delivering the power output of said engine, andsaid control comprises means, responsive to the temperature ("5 of theair entering said first compressor, for automatically controlling thecorrected peed of said second gas turbine, so that the torque exerted bysaid free power turbine is independent of said (T temperature.

17. A fuel control as in claim 15, wherein said control include meansfor obtaining maximum acceleration of the engine from one manual powerlever setting to another, without causing compressor surge and enginevertemperature.

18. A fuel control as in claim 15, wherein said fuel control comprisesmeans for modifying the fuel flow to said first combustion chamber, soas to obtain maximum engine deceleration from any manual power leversetting to any lower setting, without causing burner blowout.

19. A fuel control as in claim 15, wherein said control comprises meansto automatically compensate the fuel flow to said first combustionchamber, for variations in the temperature and pressure of the airentering said first compressor, and for changes in temperatureconditions in said recuperator.

20. A fuel control as in claim 15, wherein the fuel supply to the enginecomprises a primary fuel supply system for supplying fuel to the firstcombustion chamber, and a secondary fuel supply system for supplyingfuel to the second combustion chamber to supplement said primary fuelsupply; each system comprising a plurality of component, coordinatedhydraulic devices, arranged therein in fuel fiow series relation, andsubject to the manual power control lever, for regulatin the fuel flowtherethrough to the engine; said devices in each system beingcollectively responsive respectively to the corrected speed andcorrected fuel flow through its system; corrected speed being the actualspeed of a compressor rotor divided by the square root of the temerature of the air entering sai compressor; and corrected fuel flowbeing the actual r1161 flow to the related combustion chamber,multiplied by the pressure of the air entering the compressor supplyingair to said chamber, and divided by the square root of the temperatureof said air.

21. A fuel control as in claim 29, wherein the primary fuel systemcomprises a series of coordinated devices that collectively measurelow-pressure compressor inlet air absolute temperature, high-pressurecompressor discharge pressure, recuperator discharge temperature, andengine speed (rpm); and position a primary fuel metering valve, inaccordance with a preselected, composite function of said temperatures,pressure and speed, while the pressure drop across said valve ismaintained at a constant selected value.

22. A fuel control as in claim 21, including override engine speed andtemperature control devices, which modify the fuel flow to the engine,so as to prevent the engine operatin at excessive speeds andtemperature.

23. In operative association with a two-spool, gas turbine engine,comprising: a first rotor, consisting of a low pressure air compressordriven by a first gas turbine; at second rotor, consisting of a highpressure air compressor driven by a second gas turbine; said compressorssupplying compressed air through a heat recuperator to a first fuelcombustion chamber; and a second fuel combustion

1. IN OPERATIVE ASSOCIATION WITH A GAS TURBINE ENGINE, HAVING ACOMPRESSOR SUPPLYING COMPRESSED AIR THROUGH A HEAT RECUPERATOR TO FUELCOMBUSTION CHAMBER, A FUEL CONTROL COMPRISING: MEANS SUPPLYING FUEL TOSAID COMBUSTION CHAMBER, AT A RATE VARYING IN ACORDANCE WITH THE MASSRATE OF AIR FLOW THERETO, AND MEANS FOR AUTOMATICALLY MODIFYING THE RATEOF SAID FUEL SUPPLY, IN ACCORDANCE WITH A FUNCTION OF THE PRODUCT OF THECOMPRESSOR DISCHARGE PRESSURE AND THE TEMPERATURE OF THE AIR LEAVINGSAID RECUPERATOR, SO THAT THE TEMPERATURE OF THE GASES LEAVING SAIDCOMBUSTION CHAMBER NEVER EXCEEDS A PRESELECTED MAXIMUM LIMIT.
 14. INOPERATIVE ASSOCIATION WITH A GAS TURBINE ENGINE, COMPRISING: A FIRST,LOW-PRESSURE AIR COMPRESSOR, DRIVEN BY A FIRST GAS TURBINE; A SECOND,HIGH-PRESSURE AIR COMPRESSOR, DRIVEN BY A SECOND GAS TURBINE; FORSUPPLYING COMPRESSED AIR THROUGH A HEAT RECUPERATOR TO A FIRST FUELCOMBUSTION CHAMBER; AND A SECOND FUEL COMBUSTION CHAMBER, INTERPOSED INGAS FLOW SERIES RELATION BETWEEN SAID FIRST AND SECOND TURBINES; ASINGLE-UNIT FUEL CONTROL, COMPRISING: A MANUAL POWER CONTROL LEVER,MEANS FOR COORDINATELY REGULATING FUEL FLOW FROM A COMMON SUPPLY SOURCETO SAID FIRST AND SECOND COMBUSTION CHAMBERS, SO AS TO MAINTAIN